The planar lightwave circuit (PLC), primarily the quartz-type one, has been increasingly developed and put to practical use, and plays an important role as an essential component in bolstering the recent optical communication market including AWG (Arrayed Waveguide Grating) and splitters. Moreover, in recent years, elements having new functions, such as the hybrid wavelength variable light source the semiconductor optical amplifier (SOA) of which is mounted on the quartz PLC, has been developed. Attempts are vigorously made to realize a smaller and more inexpensive system on one chip than before by mounting the active and passive devices on the common PLC substrate.
However, as the functions required become more complex and sophisticated, the size of the elements of the planar lightwave circuit increases and more power is required to drive the planar lightwave circuit, coming close to the limits in function and performance of the conventional quartz type. Accordingly, there is progress in the research and development of SOI (Silicon on Insulator) waveguides with the use of Si microfabrication techniques involving a Si wire and a photonic crystal (PC), and the possibility of a small, low-power-consumption, and low-cost essential component is being examined.
In particular, the Si wire is an optical waveguide that can dramatically reduce the size of the conventional PLC. In downsizing the PLC with the use of the SOI waveguide, Si is used as the core material to increase the relative refractive index difference with respect to the clad material (SiO2 or the dielectric material thereof), thereby realizing a minute optical circuit.
The relative refractive index difference Δ of the conventional quartz type is just around 5% and the bend radius thereof is about 500 μm, whereas Δ of the Si wire optical waveguide is more than 40% and the bend radius thereof is several microns. However, an increase in the relative refractive index difference requires the diameter of the core to be smaller to meet the single mode condition of the propagation light, leading to a difference in spot size between the optical fiber and other optical waveguide elements. Therefore, the problem is that optical coupling losses increase.
In order to solve the problem, several methods have been proposed to enlarge the spot size. As proposed in Patent Document 1, the simplest method is a widely known method to decrease only the width of the waveguide. However, since the thickness of the waveguide remains unchanged, the field of the propagation light is elliptical in shape. Therefore, the coupling losses with the optical fiber cannot be sufficiently reduced. On the other hand, also proposed is a method to taper the thickness of the waveguide as well. Proposed in Patent Document 2 or 3 is a method to form the thickness of the waveguide independently of the width of the waveguide. However, the method is a combination of a plurality of processes, leading to complicated production processes. If the width and thickness of the waveguide core layer can be changed in the same process and at the same time, the production processes can be simplified and manufacturing variations can be suppressed, making it possible to realize the low-cost spot size conversion with high yields.    Patent Document 1: JP-A-2004-151700    Patent Document 2: JP-A-2005-70557    Patent Document 3: JP-A-2002-303752